In order to track whether a recipient opens an email, by default Propeller inserts an invisible image into the message.
In order to track whether a recipient clicks on any links in your email, by default Propeller rewrites the links to go to an https://app.propellercrm.com address first, and then redirects to the original link.
You can turn this behavior off for an individual email by clicking on the Tracking button in the compose window:
If you want to turn tracking off by default, go to the extension settings and set Track by Default to off:
If you have tracking off by default, you can turn it on for an individual email by clicking on the Not Tracking button: